Pressure-sensitive recording paper sheets, heat-sensitive recording paper sheets and the like are adapted to effect copying and/or recording by making use of color-producing reactions which respectively take place by causing electron-donating dyestuff precursors and electron-attracting acidic color-developing agents to contact with each other by pressures or heat. Reflecting recent rationalization of office work and increased utilization of facsimile facilities, these pressure-sensitive recording paper sheets, heat-sensitive recording paper sheets and the like have found wide-spread commercial utility.
In each of pressure-sensitive copying paper sheets which are widely used these days, the coated side of a sheet (a), which has been prepared by dissolving a dyestuff precursor such as triphenylmethanephthalide- or fluoran-type dyestuff precursor in a hydrophobic solvent, microencapsulating the thus-obtained solution in a manner known per se in the art and then coating the resultant microcapsules on a base material such as paper sheet or the like, is brought into a contiguous relation with the coated side of another sheet (b) bearing an oil-absorbing or oilsoluble acidic color-developing agent such as acid clay, a phenol-formaldehyde polymer or a metal salt of a salicylic acid derivative coated thereon, some of the microcapsules are ruptured by writing pressures or the like to cause the solvent, which contains the chromogenic reactant, to transfer to the surface which bears the color-developing agent, thereby allowing the chromogenic reactant and color-developing agent to undergo a reaction so that a color mark is obtained. On the other hand, heat-sensitive recording paper sheets, especially, heat-sensitive recording paper sheets generally called two-components color production systems are those obtained individually by separately dispersing a dyestuff precursor such as triphenylmethanephthalide-type or fluoran-type dyestuff precursor and an acidic phenol-type color-developing agent such as 2,2-bis(4'-hydroxyphenyl)propane (bisphenol A) or benzyl p-hydroxybenzoate into fine particles, adding a binder and other additives to the thus-prepared particulate dyestuff precursor and color-developing agent, and then applying the resultant coating formulation to a base material such as paper sheet or the like in such a way that the particulate dyestuff precursor is isolated from the particulate color-developing agent and vice versa. Color marks can be obtained through a reaction which takes place when either one or both of the particulate dyestuff precursor and color-developing agents are molten by heat to bring them into mutual contact.
There has also been disclosed a material which makes use of a dyestuff precursor as a temperatureindicating material along with an acidic substance and another substance which melts at a desired temperature. When the temperature reaches the desired temperature, the latter substance is molten and dissolves the dyestuff precursor and acidic substance so that they are brought into contact and hence allowed to undergo a reaction to produce a color, whereby indicating the temperature (Japanese Patent Laid-open No. 188528/1984).
Pressure- and heat-sensitive recording paper sheets, which are used widely and make use of color-producing reactions between phthalide- or fluoran-type dyestuff precursors and acidic color-developing agents as described above, however involve serious defective problems. Namely, (1) they require the dyestuff precursors and acidic color-developing agents in large amounts, resulting in a still-standing problem from the economical viewpoint. (2) The color fastness levels of produced color marks are insufficient so that these color marks are easily discolored, faded or even vanished upon exposure to light, heat or polar solvents.
With a view toward solving these problems, the present inventors have already proposed a novel color-producing system relying upon an oxidative color-producing mechanism which is totally different from conventional color-producing mechanisms (Japanese Patent Laid-open No. 107882/1982).